The present invention relates to measurement of two dimensional plane displacement, and more particularly to a method and apparatus for precisely measuring two dimensional plane displacement using moire fringes of concentric circle gratings.
In principle, a pair of optical gratings having fine periodic or quasi-periodic grids produce moire fringes of a high visibility when the pair of optical gratings are overlapped. The moire fringes have a frequency equivalent to a difference of spatial frequency between the optical gratings and are sensitively changed according to the relative one- or two- dimensional plane displacement induced between gratings.
Recently, with increasing demands on precise alignment between the pattern master and wafer for integrated circuits fabrication, much attention has been devoted to the measurement of the plane displacement. So, it has previously been proposed to provide an optical linear scale for measuring one dimensional displacement or to provide a rotary encoder for measuring rotational angle.
However, there is a problem that the optical linear scale or the rotary encoder cannot precisely measure the one-dimensional displacement of a body.
Accordingly, it is an object of the present invention to solve the above mentioned problem.
It is a purpose of the present invention to provide a method for precisely measuring the relative two- dimensional plane displacement of a moving body by forming moire fringes induced between two concentric circle gratings each having a different pitch.
It is another purpose of the present invention to provide an apparatus for precisely measuring the relative two- dimensional plane displacement of the body by processing an image of moire fringes induced between two concentric circle gratings each having a different pitch.